Historically, ethylene polymers used for oriented films, fibers, monofilaments, and tapes typically were unimodal polyethylenes having a high density, e.g., above 0.945 gm/cm3.
More recently, bimodal or multimodal blends of specific ethylene polymers have been proposed for film and fiber applications, with a variety of different component compositions and physical properties. For example, EP 696300 and U.S. Pat. No. 6,316,549 (Chum et al.) teach the manufacture of films and fibers from blends of one or more homogeneously branched ethylene polymers with one or more heterogeneously branched ethylene polymers. Those blends can be prepared in situ (e.g., in a multiple-reactor polymerization process) or by blending separately polymerized ethylene polymers. Similarly, U.S. Pat. No. 5,844,045 (Kolthammer et al.) teaches such blends can be made in situ in a multi-stage polymerization process.
U.S. Pat. No. 6,015,617 (Maugans et al.) teaches the use of various multimodal blends of specific ethylene polymers (such as metallocene-catalyzed homogeneous ethylene copolymers having a density<0.90 gm/cm3, with a Ziegler-Natta-catalyzed heterogeneous ethylene polymer having a density of 0.935 to 0.955 gm/cm3) can improve the bonding performance of the films or fibers or rotomolded articles made from such blends.
EP 898586 (Jacobson et al.) teaches the manufacture of films and other fabricated articles from a broad range of blends made from (a) certain metallocene-catalyzed ethylene copolymers having a narrow molecular-weight distribution and a unique comonomer-content distribution, and (b) one or more other ethylene-based polymers, such as Ziegler-Natta catalyzed polymers and/or metallocene-catalyzed linear or substantially linear ethylene polymers.
EP 1378592 (Slootweg et al.) and WO 2008/098905 (Buriani et al.) teach certain bimodal polymeric blends for use in the production of synthetic fibers for artificial turf, in which the blends comprise a first polyethylene or polypropylene component and a second, metallocene-catalyzed plastomer component (such as a metallocene or single-site catalyzed polyolefin).
WO2006/053709 (Lambert et al.) teaches drawn tapes and fibers made from blends of one component having a density of 0.91 to 0.95 gm/cm3, and a second component having a density>0.955 gm/cm3. EP 1833907 (Lambert et al.) teaches the manufacture of tapes, films and artificial turf from bicomponent blends of Ziegler-Natta-catalyzed components—one having a density of 0.92 to 0.94 gm/cm3, and the other having a density of 0.937 to 0.972 gm/cm3.
EP 1972703 (Van Pardon et al.) teaches the manufacture of films, tapes and monofilaments with blends (having an overall density of 0.915 to 0.938) made from a Ziegler-Natta-catalyzed components, one called znLLDPE and the other called znPE. Similarly, EP1972704 (Van Pardon et al.) teaches the manufacture of films, tapes and monofilaments with blends made from metallocene-catalyzed components, one called mLLDPE and the other called mPE, one of which may have a density from 0.900 to 0.930 gm/cm3, and the other having a density from 0.930 to 0.980 gm/cm3. These patents also discuss an optional “prepolymer” component, for the description of which both patents refer to WO9618662.
WO90/03414 (Stehling et al.) teaches various bimodal and trimodal blends of linear ethylene polymers having very narrow molecular weight and composition distributions. The blend components may have a variety of densities, such as in the ranges of 0.85 to 0.90 gm/cm3, from 0.900 to 0.915 gm/cm3, from 0.915 to 0.940 gm/cm3, and above 0.940 gm/cm3.
Additional polymer compositions, films and/or coatings are described in the following references: U.S. Pat. No. 5,986,028, and International Publication Nos. WO 2006/091310, WO 2004/026955 and WO 1998/21276.
Unfortunately, the performance of the incumbent ethylene polymers has been unsatisfactory in certain oriented article applications, such as artificial turf. Linear low density polyethylene (LLDPE) at densities below about 0.925 gm/cm3, when extruded into oriented tapes or fibers, typically exhibit high shrinkage values, such as 10 percent or greater. That shrinkage often results in performance problems for the end-use of such tapes, such as the monofilament fibers for artificial turf applications, where thermally induced elastic-memory results in curling and/or twisting of the fibers. This shrinkage problem is exacerbated when a metallocene-catalyzed, elastomeric ethylene polymer is blended with the LLDPE for the purpose of improving the elastic recovery (or resiliency) of the fibers.
Accordingly, there is a continuing need to develop polymers which can be formed into oriented articles having a highly desired combination and balance of properties (e.g., lower shrinkage and therefore, higher dimensional stability and lower elastic-memory effects, while also maintaining or improving tenacity and residual elongation) compared to incumbent ethylene-based polymers.
We have invented certain multimodal ethylene polymer compositions that unexpectedly improve the shrinkage performance, as well as improving the tenacity and residual elongation performance of oriented articles made from those compositions.